The embodiments relate to receivers. More particularly, the embodiments relate to scaling apparatuses, which operate in a symbol-by-symbol manner and are set in an OFDM receiver.
A receiver comprises a variable gain amplifier that adjusts the amplitude of a received signal before the received signal is fed into an analog-to-digital converter (ADC). If the received signal enters the ADC with too large amplitude, saturation will occur, causing the output of the ADC to be distorted. If, on the other hand, the received signal enters the ADC with insufficient amplitude, the signal-to-noise ratio (SNR) will become lower and the performance of the receiver will be deteriorated. Generally speaking, the process of using a control signal to adjust the gain of the variable gain amplifier is called Automatic Gain Control (AGC). Normally, the AGC process is a feed back control process. The feed back control process involves measuring the amplitude of an output signal of the variable gain amplifier, comparing the measured value with a desired value to generate a control signal, and controlling the gain of the variable gain amplifier according to the control signal. Ideally, after performing the AGC process, the amplitude of the received signal to be fed into the ADC will lie in an appropriate range suitable for the ADC.
After the ADC converts the received signal into digital domain, some interference caused by adjacent frequency bands will still exist in the digital signal. A filter is therefore required to sift out the unwelcome signal components; however, the filter also causes the desired signal to become smaller. A common solution to this problem is to set a digital AGC at the output end of the filter. The digital AGC adjusts the amplitude of the filtered signal so that the following digital signal processing procedures can be facilitated.
Some typical receivers, however, such as orthogonal frequency division multiplexing (OFDM) receivers, sometimes have to receive signals while the receiver or transmitter moves at a high speed. The channel condition between the transmitter and the receiver may change rapidly when the receiver or the transmitter is moving at a high speed. Performing the AGC process and the digital AGC process may not be sufficient for adjusting the amplitude of the signal within the preferred range in such a rapid changing channel. The performance of the receiver is therefore deteriorated when the receiver is moving at a high speed.